Transition from spot to faculae domination -- An alternate explanation for the dearth of intermediate \textit{Kepler} rotation periods

TL;DR

This paper investigates stellar activity cycles, revealing a transition from spot to faculae dominance around the Vaughan-Preston gap and proposing an explanation for the Kepler rotation period dearth.

Contribution

It introduces a method combining photometric and chromospheric data to analyze activity signatures and identifies the transition point from spot to faculae dominance in stellar activity.

Findings

Active stars are dominated by dark spots with anti-phase variability.

Inactive stars are dominated by bright faculae with in-phase variability.

The transition from spot to faculae dominance occurs at the Vaughan-Preston gap and Rossby number ~1.

Abstract

The study of stellar activity cycles is crucial to understand the underlying dynamo and how it causes activity signatures such as dark spots and bright faculae. We study the appearance of activity signatures in contemporaneous photometric and chromospheric time series. Lomb-Scargle periodograms are used to search for cycle periods present in both time series. To emphasize the signature of the activity cycle we account for rotation-induced scatter in both data sets by fitting a quasi-periodic Gaussian process model to each observing season. After subtracting the rotational variability, cycle amplitudes and the phase difference between the two time series are obtained by fitting both time series simultaneously using the same cycle period. We find cycle periods in 27 of the 30 stars in our sample. The phase difference between the two time series reveals that the variability in fast rotating active stars is usually in anti-phase, while the variability of slowly rotating inactive stars is in phase. The photometric cycle amplitudes are on average six times larger for the active stars. The phase and amplitude information demonstrates that active stars are dominated by dark spots, whereas less active stars are dominated by bright faculae. We find the transition from spot to faculae domination at the Vaughan-Preston gap, and around a Rossby number equal to one. We conclude that faculae are the dominant ingredient of stellar activity cycles at ages >2.55 Gyr. The data further suggest that the Vaughan-Preston gap can not explain the previously detected dearth of Kepler rotation periods between 15-25 days. Nevertheless, our results led us to propose an explanation for the rotation period dearth to be due to the non-detection of periodicity caused by the cancellation of dark spots and bright faculae at 800 Myr.